Eye tracking and the measurement of eye response to visual stimulus has applications in many fields including public safety, employee monitoring, computer gaming and computer interfaces for the disabled. It is well known that drug use or alcohol impairs the reaction time of an individual. This reaction time is apparent in the saccades eye movements and in the speed at which the pupils of the impaired person contract.
Saccades are the principal method for moving the eyes to a different portion of a scene. They are sudden, rapid movements of the eyes. While saccades can be initiated voluntarily, once initiated (with a path and terminal point) they must complete the process; thus the path and terminus cannot change “on the fly” during the motion. The delay from stimulus to initial eye motion is in the range of 100-300 ms. Eye motion time is in the range of 30-120 ms and dependent on the angle to be traversed. It is noted that the visual image is suppressed during the saccade, and the retina processes images only during the non-motion time (dwell time) between saccades which may last from 200 to 600 ms.
Pursuit motion can be described as a motion that keeps a moving object foveated (i.e. in the high resolution area of the retina). Contrasted to a saccade motion pursuit motion is smoother and slower. Pursuit motion, however, requires a moving object in the field of vision and cannot be generated voluntarily.
An article by L. Schovanec entitled “Ocular Dynamics and Skeletal systems,” IEEE Control Systems Magazine, August 2001, pp 70-79, describes various models of ocular dynamics along with references. For discussion purposes, one eye plant model for horizontal movement can support saccadic, pursuit, vestibular, vergence or optokinetic.
For public safety applications, a measurement of saccadic velocity, pupil diameter, pupil latency constriction, and constriction amplitude are used by the Fit 2000 device, manufactured by Pulse Medical Instruments Inc., to determine whether an individual is “fit for duty.”. This device takes a series of measurement including pupilary response and saccadic motion. The system accumulates statistical data from personnel who are deemed fit for duty and, when sufficient measurements have been acquired, it compares new measurements against the stored statistics to determine if the individual falls into or out of the range deemed fit for duty.
This device, however, is rather costly and requires a certain amount of user habituation. In many cases the device will not be able to make measurements if the user does not exactly follow the lights by focusing on a green led either at the left or right. This device measures pupil diameter at a 60 Hz rate while eye position is measured at 600 Hz.
It has also been recognized that eye motion may be used for a computer interface. It may be used as an auxiliary input channel, for example, to replace a pointing device such as a mouse, or it may be used as the primary input device by a disabled person. The use of eye motion as a computer interface is described in an article by R. J. K. Jacob entitled “Eye Movement-Based Human-Computer Interaction Techniques: Toward Non-Command Interfaces,” available at the web site of the NEC Research Institute CiteSeer as jacob93eye.html.
The system described in this paper uses a video camera with infrared illumination to track motion of the eye using an image of the cornea plus a “bright-eye” effect generated by a reflection from the retina through the dilated pupil. This apparatus is relatively large and, due to the relatively slow rate of the video camera (e.g. 30 frames per second), may not be able to accurately track eye motion.